For other uses, see Tears (disambiguation) a) Lacrimal gland

b) Superior lacrimal punctum

c) Superior lacrimal canal

d) Lacrimal sac

e) Inferior lacrimal punctum

f) Inferior lacrimal canal

g) Nasolacrimal canal Anatomy of lacrimation, showing Tearing (), lacrimation, or lachrymation (from Latin lacrima, meaning 'tear') is the secretion of tears, a body fluid which often serves to clean and lubricate the eyes in response to an irritation of the eyes.[1] Tears formed through crying are associated with strong internal emotions, such as sorrow, elation, love, awe, and pleasure. Laughing or yawning may also lead to the production of tears. Contents

Physiology Edit

In humans, the tear film coating the eye, known as the precorneal film, has three distinct layers, from the most outer surface:[2] Name Container(s) Secretors Functions Lipid layer Oils Meibomian glands (or tarsal glands) Coats the aqueous layer, provides a hydrophobic barrier that envelops tears and prevents their spilling onto the cheek. These glands are found among the tarsal plates. Thus, the tear fluid deposits between the eye proper and oil barriers of the lids.[3] Aqueous layer Electrolytes, 60 metabolites—Amino Acids (1-Methylhistidine/3-Methylhistidine, Arginine, Asymmetric, Asymmetric dimethylarginine/Symmetric dimethylarginine, Citrulline, Creatine, Glutamine, Homoarginine, Hydroxyproline, Phenylalamine, Proline, Pyroglutamic acid, Serine, Taurine, Theonine, Tryptophan, Tyrosine, Urocanic acid, Valme), Amino Alcohols (Panthenoll); Amino Ketones (Allantoin, Creatine), Aromatic Acids (Cinnamic acid, o-Coumaric acid/m-Coumaric acid/p-Coumaric acid), Carbohydrates (N-Acetylneuraminic acid), Carnitines (Acetylcarnitine, Carnitine, hexanoylcarnitine, Palmitoylcarnitine); Cyclic Amines (Niacinamide); Dicarboxylic Acids (Fumaric acid/ Maleic acid), Nucleosides (1-Methyladenosine, Adenoisine, Cytidine, Guanosine, Inosine, S-Adenosyl-homocysteine, S-Adenosyl-methionine, Uridine, and Xamthosine), Nucleotides (ADP, AMO, CMP, Cytidine diphosphate choline, GMP, IMP, UDP, UMP, UDP-N-acetylgalactosamine/UDP-N-acetylglucosamine), Peptides (Oxidized glutathione), Phospholipids (1-Palmitoyl-lysophosphatidylcholime), Purines and derivatives (Hypoxanthine, Theobromine, Uric acid, Xanthine), Purines and derivatives (4-Pyridoxic acid), Quaternary Amines (Acetylcholine, Glycerolphosphocholine, Phosphocholine), and Tricarboxylic Acids (Citric acid)--and other substances such as proteins (e.g., antibodies,[2] lipocalin, lactoferrin, lysozyme,[4] and lacritin) Lacrimal gland Promotes spreading of the tear film, the control of infectious agents and osmotic regulation. Mucous layer Mucins Conjunctival goblet cells Coats the cornea, provides a hydrophilic layer and allows for even distribution of the tear film. Having a thin tear film may prevent one's ability to wear contact lenses, as the amount of oxygen needed is higher than normal, and contact lenses stop oxygen from entering the eye. Eyes with thin tear film will dry out while wearing contact lenses. Special eye drops are available for contact lens wearers. Certain types of contact lenses are designed to let more oxygen through to the eye.

Drainage of tear film Edit

The lacrimal glands secrete lacrimal fluid, which flows through the main excretory ducts into the space between the eyeball and lids. When the eyes blink, the lacrimal fluid is spread across the surface of the eye. Lacrimal fluid gathers in the lacrimal lake, and is drawn into the puncta by capillary action, then flows through the lacrimal canaliculi at the inner corner of the eyelids entering the lacrimal sac,[3] then on to the nasolacrimal duct, and finally into the nasal cavity. An excess of tears, as with strong emotion, can thus cause the nose to run. [1]

Types Edit

Diseases and disorders Edit

Social aspects Edit

Crying boy In nearly all cultures, crying is associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most often sadness and grief but crying can also be triggered by anger, happiness, fear, laughter or humor, frustration, remorse, or other strong, intense emotions. In many cultures, crying is associated with babies and children. Some cultures consider crying to be undignified and infantile, casting aspersions on those who cry publicly, except if it is due to the death of a close friend or relative. In most cultures, it is more socially acceptable for women and children to cry than men. In some Latin regions, crying among men is acceptable.[11][12][13] Some modern therapy movements such as Re-evaluation Counseling teach that crying is beneficial to health and mental well-being, encouraging it positively.[14] An insincere display of grief or dishonest remorse is sometimes called crocodile tears in reference to an Ancient Greek anecdote that crocodiles would pretend to weep while luring or devouring their prey.[15] In addition, in medical terms, someone is said to have crocodile tears syndrome as an uncommon consequence of recovery from Bell's palsy, in which faulty regeneration of the facial nerve causes sufferers to shed tears while eating.

Chemicals in tears Edit

On a study conducted by the Weizmann Institute of Science in Rehovot, Israel, emotional tears from women have been found to reduce sexual arousal in men. Also, emotional tears are made up of a different chemical component than those evoked by eye irritants and can relay chemical messages to others.[16] The change in sex drive could be attributed to a drop in testosterone provoked by the tear chemicals, reducing aggression. In the animal world, it has been found that some blind mole rats rub tears all over their bodies as a strategy to keep aggressive mole rats away.[17] Tear composition varies from tear types. Mainly, tears are composed of water, salts, antibodies and lysozymes (antibacterial enzymes). According to a discovery by Dr. William H. Frey II, a bio-chemist from St. Paul Ramsey medical center in Minnesota, the composition of tears caused by emotion differs from that of tears as a reaction to irritations, such as onion fumes, dust or allergy. Emotional tears are composed of more protein-based hormones, such as prolactin, adrenocorticotropic, and leucine enkephalin (a natural pain killer), which is suggested to be the mechanism behind the experience of crying from emotion making an individual feel better.[18]

See also Edit